The present invention relates to an x-ray system and method for identifying material within an obscuring enclosure, and more particularly to a system and method using a combination of side-scattered and coherently-scattered penetrating radiation for discriminating target materials.
The angular distribution of x-ray radiation scattered from a material when the radiation incident on the material is substantially monochromatic provides a well-established method for identifying the scattering material. The basis of the identifying characteristics of the scattered radiation is coherent x-ray scattering from the crystal planes of the bulk material. The well-known Bragg equation governs this so-called wavelength dispersive spectroscopy:
sin xcex8=nxcex/(2d)xe2x80x83xe2x80x83(1)
where d is the spacing between crystal planes, xcex8 is the scattering angle, n is the order of scattering and xcex is the wave length of the radiation. Practitioners typically use low energy x-rays for these measurements, for example, the 8 keV (1.5 xc3x85) x-rays from copper produce strong Bragg peaks at large, easily measured, scattering angles.
However, the identification of material in the interior of large containers typically employs radiation of higher energy. In particular, for luggage brought on board aircraft, typical x-ray energies are at least 75 keV, corresponding to a wavelength of ⅙th of an Angstrom. At this energy, the first Bragg peak (the closest to xcex8=0xc2x0) will then be at a very small angle, typically in the range of a few degrees, making wavelength dispersive spectroscopy extremely difficult.
A more practical approach for the use of coherent-scattering at higher energies, suggested by G. Harding and J. Kosanetzky, xe2x80x9cScattered X-Ray Beam Non-Destructive Testing,xe2x80x9d in Nuclear Instruments and Methods (1989), is to use energy dispersive spectroscopy. In energy dispersive spectroscopy, a polychromatic beam of high energy x-rays is sent through the container and the energy distribution at a fixed scattering angle of a few degrees is used to identify the object. The governing equation is the same as Eqn. 1, written to emphasize the energy dependence:
E={6.2}/{d sin xcex8}≈{6.2}/{dxcex8},xe2x80x83xe2x80x83(2)
where d is the crystalline spacing in Angstroms, xcex8 is the scattering angle in radians, and E is the x-ray energy in keV. Thus, for example, an x-ray of 100 keV will be Bragg scattered through an angle of about 2xc2x0 by a crystalline substance with spacings of about 2 xc3x85.
Bragg-scattering inspection systems under current development seek to examine the entire volume of every piece of luggage that enters an aircraft. The hardware to carry out this daunting task is complex and expensive, and is at least 2 orders of magnitude too slow to be effective as a screener at an airport terminal.
Additionally, since the Bragg scattering angles are so low (typically 2xc2x0-3xc2x0), the collimation requirements on the detector are stringent if a particular volume along the x-ray path into the interrogated volume is to be discriminated. The strict requirement on the collimation of the coherent-scatter detector can be quantified by noting that an uncertainty in the angle results in an uncertainty in the measured energy. Differentiating Equation (2) gives the necessary relation:
xcex94E/E≈xcex94xcex8/xcex8.xe2x80x83xe2x80x83(3)
To obtain a full-width energy resolution of xcex94E/E=5%, the angular uncertainty xcex94xcex8/xcex8 must be kept to 5%. (A 5% uncertainty is typical of the maximum uncertainty that can be tolerated if the coherent-scatter method is to effectively discriminate between different types of materials.) The collimation must therefore be good enough to limit the acceptance angle to 2xc2x0 with an accuracy of 5%, a difficult requirement.
The small scattering angles with their tight uncertainty requirements severely restrict the length along the beam that can be inspected by a single coherent-scatter detector, typically to no more than 3 cm. If the position along the beam path of a suspect volume of an inspected enclosure is unknown, then it becomes necessary to make 5 to 10 separate measurements (or, alternatively, to provide the same number of carefully collimated detector elements) to inspect all the voxels (i.e., volume elements) along a given beam path. In one case, inspection times are increased, and in the other, the cost of the system is impacted substantially.
In a first embodiment of the invention there is provided a method for inspecting an enclosure. In accordance with the method, an enclosure is irradiated with penetrating radiation, radiation side-scattered from an object within the enclosure is detected, and the object is located. On the basis of the side-scattered radiation, a decision is made as to whether the object is suspect. If the object is deemed suspect, a volume element of the suspect object is further irradiated with penetrating radiation. Radiation coherently-scattered by the volume element is detected. The energy spectrum and angular distribution of the coherently-scattered radiation then are used to characterize the volume element of the suspect object.
In accordance with another embodiment of the invention, determination of whether an object is suspect is determined on the basis of the mass density of the object. The mass density is derived from side-scattering, including Compton scattering, where the enclosure is scanned with an x-ray beam generated by a wheel with a plurality of hollow spokes and an x-ray source at its center.
In accordance with a further embodiment of the invention, an object is identified on the basis of mass densities of contiguous volume elements of the suspect object. A volume element of the suspect object is irradiated by aligning a single pencil-beam collimator with an x-ray source and with the volume element of the suspect object.
In accordance with still further embodiment of the invention, the enclosure containing the object is transported on a conveyor belt through an irradiating beam. The radiation side-scattered by the object is subsequently detected. The conveyor belt may then be halted and a volume element of the suspect object irradiated. Coherently-scattered radiation is subsequently detected. As the conveyor belt is halted, detection of objects continues. Alternatively, coherently scattered radiation may be detected without halting the conveyor belt.
In accordance with still another embodiment of the invention, irradiation of an enclosure and irradiation of a volume element of a suspect object are performed with the same source of radiation. The source of radiation moves in the conveyor direction from its location during irradiation of the enclosure to its location during irradiation of a suspect object.